The field of the invention is optics, measuring and testing the velocity or velocity/height with light detector. The present invention relates to a method and apparatus for photoelectrically determining the position of at least one focal plane of an image within an optical instrument, together with the imaging of at least one object onto a spatial frequency filter of an optical image correlator and the measurement and/or indication of the light fluxes leaving the spatial frequency filter.
Such a method and apparatus are described in U.S. patent application Ser. No. 479,525 of Leitz et al, wherein light fluxes passing through different pupil regions of the imaging optics are modulated in common by the spatial frequency filter and are split geometrically or physically depending on the pupil region or by additional modulation, and then are fed sequentially to a common photoelectric detector or simultaneously to separate ones, where the output signals are further processed for the purpose of control of an indication and/or adjustment system. Further, for the purpose of control with respect to algebraic sign of an indication and/or control system, the magnitude and/or the phase for the case of relative motion between spatial frequency filter and image and/or the frequencies of the incident electrical signals may be determined. Again, when making use of spatial frequency filter systems generating out-of-phase light fluxes, the light fluxes passing through the different pupil regions of the optics may be fed following their splitting as out-of-phase pairs in sequence or simultaneously to a common photoelectric pair of detectors where the output signals from each detector pair associated with a particular pupil region are applied to a particular push-pull amplifier, the output signals from the latter thereupon being compared for the purpose of algebraic signal control of an indication system and/or adjustment system regarding the magnitude and/or, if there is relative motion between spatial frequency filter and image, the relative phase or frequency.
It was found that the signals obtained from the method and apparatus of application Ser. No. 479,525 are well suited for measuring distance and for adjusting control signals. However, it was also found that the method and apparatus of application Ser. No. 479,525 unequivocally ascertains the position of the image plane or the distance within a limited range. This is so because there is a perodicity in the analysis signals, which leads to ambivalence. When there is motion together with the image plane within a range of position corresponding to half a signal period, then when this range is passed through there occurs only one signal maximum, and the position of the image plane may be uniquely ascertained. When a grating or grid is associated with two photoelectric detectors, and when this grating or grid traverses the imaging range of the optics, then the detector output signals for the grid or grating in the focal plane not only will be of maximum amplitude, but also they will be of phase. Depending on the direction of the displacement of the grid or grating from that plane along the optical axis, the phase of one or the other signal will lead. Now it is possible that within the image field of the object to be measured, and at some other distance, there will be another object which because of its radiation (for instance the tail lights of a motor vehicle) will deliver a higher signal amplitude than the object to be measured, and therefore, there will be spurious interpretation and measurement.
The above effects are the more disadvantageous the larger the range which one wishes to cover, and the more one passes from far distances to near distances.